Nine members originally formed the Fellowship of the Ring in Tolkien’s novel.
The Fellowship consists of Frodo Baggins, Samwise Gamgee, Merry Brandybuck, Pippin Took, Aragorn, Boromir, Legolas, Gimli, and Gandalf. They are chosen at the Council of Elrond to carry the One Ring toward Mordor and seek its destruction in Mount Doom.
The number nine deliberately balances the nine Nazgûl, the Ringwraiths who serve Sauron. The company also represents several peoples and traditions of Middle-earth: Hobbits, Men, an Elf, a Dwarf, and a wizard. Their shared mission does not erase their differences, and disagreements become important as the journey progresses.
The Fellowship is formally established at Rivendell, but it does not remain together for the whole quest. Gandalf falls in Moria, and the group breaks at Amon Hen after Boromir’s death and the capture of Merry and Pippin. The nine therefore describe the original company, not every later traveler.